Problem: $ { \left[\begin{array}{rr}{1} & {0} \\ {-1} & {-2} \\ {1} & {-1}\end{array}\right]+\left[\begin{array}{rr}{4} & {-2} \\ {1} & {-2} \\ {-1} & {-1}\end{array}\right] = {?} } $
Explanation: To find the sum of two matrices, add the corresponding items between each matrix. ${\left[\begin{array}{rr}{1}+{4} & {0}+{-2} \\ {-1}+{1} & {-2}+{-2} \\ {1}+{-1} & {-1}+{-1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{5} & {-2} \\ {0} & {-4} \\ {0} & {-2}\end{array}\right]}$